1. Field of the Invention
The present invention relates to an image forming process, to which a thermal transfer method is applied, and an image forming apparatus.
2. Description of the Related Art
A method for forming an image on a transfer material, such as paper or a film, includes a thermal transfer method. In the thermal transfer method, a transfer body (transfer sheet), which contains a support and a transfer layer comprising a pigment based dye as a transfer material, is superposed on an image receiving body (an image receiving sheet) such as paper or a film. The superposed bodies are heated imagewisely from the side of the support of the transfer sheet with, for example, a thermal head or a laser head. Due to the heat, the pigment-based dye is transferred to the image receiving sheet to form an image on the image receiving sheet. In another embodiment of the thermal transfer method, a dye provided on a transfer body is transferred to an image receiving sheet due to sublimation of the dye caused by heating.
In the thermal transfer method, those using a thermal head for imagewise heating has such a problem that the distribution of a provided temperature is liable to be nonuniform due to variation of the resistance value of the head, and as a result, an image thus formed suffers unevenness. Further, because high temperature heating is locally carried out with the thermal head only on image portions, such a problem occurs that the support on the heated portions is deformed to get wrinkled. On the other hand, in order to improve uniformity of an image, heating in the thermal transfer method can be carried out with laser light. However, it has problems such that a device for irradiating a laser light or the like is very expensive, and materials such as a dye tend to be decomposed by instantaneous local heating by high temperatures to cause unevenness on an image thus formed.
In contrast to the aforementioned image forming methods, there is a non-contact type image forming method that exerts no influence on an image receiving sheet and a coloring material which forms an image. The method includes an ink-jet method, in which an ink is discharged as droplets to form an image. Type of the ink-jet method includes various types, such as a piezo type, a thermal type and a hertz type, which are described in detail in Journal of Imaging Science and Technology, vol. 42, No. 1 (1998), U.S.A. The piezo type method will be described herein after. In the piezo type ink-jet method, plural nozzle holes with ink heads arranged in parallel, independent discharge chambers interconnected with the nozzle holes and each having a diaphragm as a part of walls, piezoelectric elements attached on the diaphragms and a common ink cavity which supplies an ink to the discharge chambers are comprised. A pulse voltage is applied to the piezoelectric elements in accordance with image information to discharge ink droplets from the nozzle holes, whereby an image is formed on an image receiving sheet.
The ink-jet method dose not involves problems such as the deformation of the support and the deterioration of the coloring material as described above. However, in order to prevent clogging of the nozzle with the ink and form uniform ink droplets in a stable manner, a number of restrictions in property of ink liquid and materials used for forming an image are required. That is, the materials for forming an image cannot be freely selected. Therefore, in the case where a color image is required to be formed, it is necessary to select particular dyes or pigments to prevent clogging of nozzles. Consequently, the hue reproducibility in images is limited, and therefore the method cannot be applied to, for example, a printer for high precision proof printing, which is required to reproduce the same hue on an image receiving sheet as a ink pigment used for printing. Furthermore, the problem of nozzle clogging caused after neglect of a long period of time still remains even when particular image forming materials which are selected carefully are used. There is a further problem that an image thus formed by the ink-jet method is poor in light resistance and water resistance, and the dye is liable to be bled on the image receiving sheet.
Japanese Patent Application Laid-Open (JP-A) No.11-70633 discloses an example of an image forming method, to which the ink-jet method is applied. In this method, droplets of a crosslinking agent are imagewisely applied to a recording layer which is formed with a crosslinkable material by an ink-jet device. An applied portion of the recording layer is hardened by crosslinking, and a non-crosslinked portion of the crosslinkable material is then removed by washing to form an image portion. This method is a method to form a so-called screen printing plate by using the ink-jet, followed by forming an image by using a colored ink with the plate. It is completely different from the transfer method of the present invention since it contains two steps, plate making and printing. Further, the method involves a problem of processing of waste liquids after washing because a developing step is necessary upon plate making, whereby complicated steps are required.
Additionally, a transfer ink-jet system is proposed, for example, in JP-A No.5-42755, in which an image is formed on an image carrying member with a recording liquid which contains a liquid and a coloring agent by an ink-jet recording system, and the image is transferred to a transfer material. In this transfer ink-jet method, an image containing a coloring agent is once formed on an image carrying member on, for example, a drum by ordinary ink-jet method, and then the image is then transferred to the transfer material. Therefore, this method is different from the present invention and involves the same problems as in the ordinary ink-jet methods, i.e., the restriction of coloring materials and the clogging of nozzles. Furthermore, the method provides only an image that is liable to be bled and is of low resolution.
An image forming method utilizing an ink-jet method using a transfer medium is described in JP-A No.7-145576. In this method, an ink-jet ink is spotted imagewisely on a transfer medium having a liquid-reactive resin layer as an uppermost layer by an ink-jet recording device. Therefore, the transfer medium is closely attached to an image supporting body (such as cloth), and heat and pressure are applied thereto, so as to transfer image portions. It is such an image forming method that is particularly possible to textile-print on cloth. For example, in the case where the ink is an aqueous ink, the liquid-reactive resin layer is formed with a water soluble resin, and when an ink droplet is spotted thereon, the resin on the spotted portion is dissolved to have cohesiveness (adhesiveness). Owing to the cohesiveness, transfer occurs only on the spotted portion by the heat and pressure. Therefore, high precision image formation becomes possible on cloth by using the ink-jet method. However, this method particularly targets cloth as an image carrying member, and still uses the ink-jet method. Accordingly, it involves the same problems as in the ordinary ink-jet methods, i.e., the restriction of coloring materials and the clogging of nozzles. Furthermore, it also involves such a problem that transfer of the resin layer cannot be carried out uniformly to cause unevenness.
Moreover, JP-A No.62-117782 describes the following image forming method. In the method, a solvent is applied imagewisely to a surface of an image carrying member by an ink-jet discharge system, and then an ink layer is made in contact with the image carrying member, whereby only the part of the ink layer that has been contacted with the imagewise solvent is formed on the image carrying member. However, as described in the right upper column of page 4, lines 7 to 16, of the publication, selection of the solvent is difficult because such a solvent is required that has suitable cohesion and adherence. Furthermore, a synthetic resin film is disclosed in the publication as the image carrying member, and it is difficult for the film to obtain good and stable transfer property because the film does not have solvent acceptability, unlike a binder coating layer. Furthermore, this method is insufficient in reproducibility of minute dots, and it is particularly impossible to obtain a multi-color transfer image of high resolution.
JP-A No.7-276780 describes a method that is an improvement of the method described in aforementioned JP-A No.62-117782. In this method, in order to apply a solvent imagewisely to a recording medium, heat energy is applied to a porous body comprising a solvent impregnated therein in accordance with image information, so as to discharge the solvent as mist (microdroplets) or vapor on the image recording medium. This method has such an advantage that clogging does not occur since the ink-jet discharge system is not used. However, this method is based on the same technique as those described in JP-A No.62-117782 as noted above except that mist or vapor is applied to the recording medium, and therefore, the same problems remain.
In order to solve the problems, the inventors of the present invention proposed an image forming method which utilizes an image transfer material containing a support having provided thereon a transfer recording layer which contains a thermal transfer material, and an image receiving sheet having an image receiving layer which contains a binder resin. In this method, a liquid containing an adhesive material or transfer-promoting material that lowers the transfer temperature of the thermal transfer material (a latent image-forming solution) is applied imagewisely and substantially to a surface of the transfer recording layer of the image transfer material or an image receiving surface of the image receiving sheet, so as to form a latent image. Thereafter, the surface of the transfer recording layer of the image transfer material and the image receiving surface of the image receiving sheet are closely in contact with each other, followed by heating, whereby the transfer recording layer corresponding to the latent image is transferred to the image receiving sheet (as described in Japanese Patent Application Nos.12-163273 and 11-288179).
According to the image forming method, the problems of the restriction of image forming material and the clogging of nozzles, which are the disadvantages of the ink-jet system, are solved, and the damage of the support due to heat and unevenness of the image, which are the disadvantages of the thermal transfer system, are also solved. Furthermore, an image which is excellent in hue reproducibility can be formed, and excellent resolution of image can be obtained. Particularly, in the case where a multi-color image is formed by the image forming method, an image can be obtained such that it is considerably excellent in comparison with the conventional transfer methods. However, it is the current situation that the demand for improving the image quality is becoming more severe, and it is desired that problems such as phenomena described below should be avoided. One phenomenon is that, when both a region where such images as highlights, microdots and/or microlines are formed and a region where images of high gradient and/or high image density are formed are provided by using the same latent image-forming solution, the latent image-forming solution tends to be insufficiently attached to the region where images such as highlights, microdots and microlines are formed, and a lack of dots and a ring stain (i.e., only a fringe of a dot becomes dense in a ring form) occur, and make the image, formation insufficient. Further, another phenomenon is that, in the case where the liquid containing the transfer-promoting material as the latent image-forming solution is used for forming a multi-color image, when the next color is to be superposed on a image-portion having high gradient (high garadatiopn) or high image density, there are some cases where unpreferable fogging (secondary fogging) occurs due to a remaining latent image-forming solution corresponding to the previous color. However, in order to avoid the secondary fogging, the load for drying the latent image-forming solution must be increased.
An object of the present invention is to further improve an image quality in the foregoing image forming method. The present invention provides an image forming process in that plural latent image-forming solutions each having concentration different from each other are discharged from plural nozzles. The process provides satisfactory image formation which can be carried out for any image condition such that the image forming region is (i) a high light portion and/or a microimage portion, or (ii) a high gradation portion and/or a high image density portion, and the image formation is carried out with high resolution setting or low resolution setting. The present invention also provides an image forming apparatus utilizing the image forming process.
A first aspect of the present invention is an image forming process comprising:
(i) preparing an image receiving sheet, an image transfer material and at least two latent image-forming solutions, the image receiving sheet comprising an image receiving layer which contains a binder resin, the image transfer material comprising, on a support, a transfer recording layer which comprises a thermal transfer material, the latent image-forming solutions respectively comprising at least one of an adhesive material and a transfer-promoting material which is capable of lowering a transfer temperature of the thermal transfer material, one of the latent image-forming solutions having a lower concentration of the at least one of an adhesive material and a transfer-promoting material than another of the latent image-forming solutions;
(ii) forming a latent image by imagewisely applying the latent image-forming solutions from a plurality of nozzles to at least one of a transfer recording layer surface of the image transfer material and an image receiving layer surface of the image receiving sheet; and
(iii) transferring the portion of the transfer recording layer that corresponds to the latent image to the image receiving sheet, by contacting and heating the transfer recording layer surface with the image receiving layer surface.
A second aspect of the present invention is an image forming apparatus for imagewisely transferring a transfer recording layer of an image transfer material to an image receiving sheet. The image transfer material comprises the transfer recording layer on a support, the transfer recording layer comprising a thermal transfer material, and the image receiving sheet comprises an image receiving layer that contains a binder resin. The apparatus comprises: a nozzle head including a plurality of nozzles supplied with at least two latent image-forming solutions and disposed to discharge the latent image-forming solutions towards at least one of a transfer recording layer of the image transfer material and an image receiving layer surface of the image receiving sheet, the latent image forming solutions respectively including at least one of an adhesive material and a transfer-promoting material which is capable of lowering a transfer temperature of the thermal transfer material, the latent image forming solutions having different concentrations; and
a pair of pressure rollers disposed on the conveyance path downstream of the nozzle head such that when the pressure rollers are rotated the image transfer material and the image receiving sheet are passed between the pressure rollers with the transfer recording layer surface and the image receiving layer surface being pressed together, at least one of the pressure rollers including heating means whose heating temperature is controllable.